Online transactions usually require online accounts. Each online account contains information related to the user of account, such as email address, name, home address, phone numbers, etc. To avoid being caught and increase success rate, malicious users usually use an account for illegal usage for a short time and then switch to another account, thereby needing a lot of accounts. The accounts used for illegal transactions may be compromised accounts of other legitimate users, or may be created by the malicious users. Automatic programs (i.e., bots) may be used to generate a large amount of email addresses to register such fraudulent accounts. For example, such fraudulent accounts may be associated with some stolen or illegal credit cards to make fraudulent transactions online. The information associated with an account, such as the associated email address, is reviewed manually for identification of malicious accounts, which is costly, labor intensive, and time consuming.